Modulation



Jan. 18, 1938. G. l.. ussELMA MODULATION original Filea sept. so, 1932 2 Sheets-Sheet l INVENTOR GEORGE L. USSELMAN A .Ian. 18, 1938.Y G, 1 Ussl-:LMAN

MODULATI ON 2 Sheets-Sheet 2 Original Filed Sept. 30, 1952 NN H i w INVENTOR GEORGE L. USSELMAN BY if ATTE) 'lfl-JYM Patented Jan. 1s, 193s NETED STATES PATENT OFFICE assigner to Radio Corporation of America, a corporation of Delaware Application September 30, 1932, Serial No. 635,508 Renewed March 4, 1936 11 Claims.

This invention relates to signalling means and in particular to means whereby the characteristics of high frequency oscillations other than amplitude are varied in accordance with signals 5 to be transmitted.

It has been found that ordinary amplitude modulated high frequency oscillations in transmission from the sending station to the receiving station are subject to what is known as fading elfects. This is a decided disadvantage since it introduces drop-outs and errors in the signal. Even where diversity receivers are used to receive the amplitude modulated signals the effect of fading is of serious disadvantage.

l5` I have found that if the high frequency oscillations are modulated in phase or in frequency in accordance with the signal to be transmitted they are less subject to the eect of fading than amplitude modulated Waves. The reason why 20 phase or frequency modulated oscillations in radio transmission are subject to a less extent to fading effects than oscillations modulated otherwise is that a greater amount of transmitter power is available for transmission. In other g5 words, a transmitter can operate at full power in phase or frequency modulation, While it is necessary to reduce the power output to about one quarter for amplitude modulation. Assuming equal power output, reduction of fading may 30 be slightly greater in phase or frequency modulation than in other kinds of modulation.

In each of my United States applications, Serial No. 623,558, filed July 20, 1932, Patent #2,048,900 dated July 28, i936, Serial No. 6l6,026,

filed June 8, 1932, Patent #2,036,165 dated March 3l, 1936, Serial No. 624,258, filed July 23, 1932,

and Serial No. 607,932, filed April 28, 1932, Patent 2,036,164, dated March 31, 1936, I have shown means for varying at signal frequency the char- 40 acteristics other than the amplitude of a carrier frequency. In each of these arrangements the carrier frequency is impressed through phase shifting means onto the control grids of a pair of thermionic tubes which have their anodes con- 45 nected in parallel to a common tank circuit and their internal impedances Varied in phase opposition by the signal wave. In each of these arrangements the phase modulator stage comprises two tubes having their input electrodes sym- 50 metrically connected and energized as indicated above, and a common tank circuit connected in parallel to the anodes of said pair of tubes.

The present invention relates to an improved and simplied modulating means by which the 55 characteristics of a carrier wave other than amplitude may be varied in accordance with the signal to be transmitted.

More in detail, this invention relates to an improved modulating means wherein the modulator stage comprises a single tube having a con- 5 trol grid connected to a source of oscillations of constant frequency by way of a phase shifting circuit having two branches of different electrical characteristics which impart to the high frequency oscillations phase shifts of the desired 10 degree and direction. The ability of the branches to transfer energy to the modulator is eiiected alternately in opposite senses by the modulating signals in such a manner that phase modulation of the carrier is effected on the grid of the modulator tube. The modulating potentials may be amplied before they are used to effect in phase opposition the alternating current conducted in the transmission lines of the phase splitting circuit between the source of high frequency oscillations and the grid of the modulator tube.

The invention, due to its simplicity and eiiciency in operation, is of considerable importance since it is applicable to new installations and to old transmitters now in use, which perhaps use other types of modulation. All that is necessary in the latter case is to remove the input circuit of the modulation tube in the transmitter and insert in place thereof the phase splitting circuit of the present invention. The signal potential amplifier of the old equipment may be used to control the alternating current conductivity of the channels of the phase splitting current, or new modulating equipment may be provided.

The novel features of my invention have been pointed out with particularity in the claims appended hereto.

The nature of my invention and the operation thereof will be best understood from the following detailed description thereof and therefrom when read in connection with the drawings, throughout which like reference numerals indicate like parts, and in Which:

Figure 1 illustrates a specific embodiment of 45 the invention; while,

Figure 2 illustrates a modication of the arrangement of Figure 1.

In describing my invention reference will be rst made to the arrangement shown in Figure 1. A constant frequency oscillator A of any known type, as, for example, a crystal or long line frequency control type oscillator, is connected through blocking condenser I, as shown, to a phase splitting circuit PS. The phase splitting circuit comprises two branches, one of which includes the phase retarding inductance 2 and resistance fl, and the other of which includes the phase advancing element 3 and resistance 5.

The phase splitting circuit PS is connected through the coupling and blocking condenser 6 to the control grid 26 of the modulator tube Z. Tube Z is of the screen grid type, but it will be understood that tubes of other types may be used. The anode electrode 28 of tube Z is connected to the tank circuit 383, as shown. The tank circuit includes a variable capacity C and an inductance I. The output or tank circuit 3d of stage C is coupled through a line including blocking condenser 3l to the radio frequency stage D, which may be either a frequency multiplier and limiter or a straight amplifier. Stage D is connected to the antenna E by the transmission line 32.

The direct current source 2l represents the power supply for the circuits and tubes, as shown. This may be of any desirable type, such asbattery, motor-generator, or vacuum tube rectifier. Heating current for the cathodes l2 and I3 of tubes X and Y and 29 of tube Z is supplied by way of leads FL from a portion of the source 2l. Direct current potential is supplied to the anodes and 9 of tubes X and Y by way of a lead I8 and to the anode 2B of tube 2 by way of a lead 22. The desired direct current biasing potential for the grids lil and l! of tubes X and Y and 2% of tube Z is supplied by leads 23 and 24 connested to movable points on potentiometer resistance P. Direct current potential for the screen grid electrode 2l of tube Z is supplied by a lead 25 connecting said electrode to a movable point on potentiometer resistance P2. Radio frequency oscillations are shunted around the source 2l by by-pass condensers C1, C2 and C3 connecting the terminal of the grid inductance IG, the screen grid electrode 2l and the terminal of inductance I to the grounded side of the cathode heating circuit FL.

The anode electrodes 8 and 9 of tubes X and Y are connected to the phase splitting circuit between inductance 2 and resistance 4 and between inductance 3 and resistance 5 respectively. The grids I0 and ll of the modulating potential modulating tubes X and Y are connected in phase opposition through the parasitic prevention resistors i4 and l5 to the terminals of the secondary winding lil of the signal frequency transformer T. The primary winding 2Q of transformer T is connected to the signal source B. Tubes X and Y are supplied with negative bias voltage from the battery 2l through rnidtap in the secondary winding I9 of transformer T and lead 23. Condensers IS and i1 act as radio frequency by-pass condensers for the circuits of grids I0 and il but they do not interfere with the signal frequency flowing in saidcircuits. The anode electrodes 8 and 9 of tubes X and Y are supplied with positive power from the source 2l through lead I8 and the inductance I and resistors 4 and 5 of the phase splitting circuit as shown.

In operation substantially constant high frequency energy is generated in the unit A. This alternating current energy of high frequency is conducted. to the phase shifting elements 2 and 3 by the coupling condenser l. Condenser l will pass alternating current but will block direct currents. The alternating current energy passing through inductive element 2 will be retarded, i. e., shifted back, by a certain phase angle and the alternating current energy passing through the capacitive element 3 will be advanced, i. e., shifted ahead, by a certain phase angle. Part of the energy from element 2 will pass through the impedance between the anode and filament of tube X to ground, while the other part of the energy from element 2 will pass through resistance element 4 and condenser 6 to the grid 2S of tube Z in stage C. Likewise, part of the energy from element 3 will pass to ground through the impedance between the anode and filament of tube Y and the other part of the energy from element 3 will pass through the resistance element 5 and the condenser to the grid 26 of tube Z in stage C. The condenser will pass alternating current energy, but will block direct current.

When no signal modulation is present each side of the phase splitting circuit will deliver alterhating current energy in equal amounts to the grid 26 of tube Z, and the resultant excitation of the grid 26 will be constant in power and phase angle.

Now if signal potentials be sent out from source B, the grids lil and l2 of tubes X and Y, being connected in phase opposition through transformer IS to the source B, will have their biasing potential changed in phase opposition according to the frequency and intensity of the signal. This will cause variation in the anode current of these tubes and consequently variations in the alternating current impedances of these tubes, which are connected in phase opposition to the phase splitting circuit PS in accordance with the frequency and intensity of the signal. This will vary, in phase opposition in accordance with the frequency and intensity of the signal, the amount of alternating current energy delivered by each side of the phase splitting circuit PS to the grid 25 of tube Z. For example, suppose grid il) of tube X is swung positively while grid ll of tube Y is swung negatively. The anode current of tube Y will be decreased. As a result, the alternating current impedance of tube X will be reduced and the impedance of tube Y will be increased. Consequently, a larger part of the alternating current energy from element in the phase splitting circuit, will pass to ground through the impedance of tube X and a smaller part of the energy from. 2 will reach the grid 2E of tube Z. Conversely, a smaller part of the alternating current energy from the phase splitting circuit element 3 will pass to ground through the impedance of tube Y and a larger part of the energy will reach the grid 26 of tube Z. The nal result is that the excitation voltage of grid 263 is advanced in phase. In the next half cycle of the signal frequency the excitation voltage to grid 2S will be retarded. If all of the elements in the phase shifting circuit and the signalling circuit are balanced the angle of phase swing of the excitation energy to control grid of tube Z will be equal on each side of the average position, and the amplitude of excitation energy to the grid of tube Z will be substantially constant. The phase swing will also vary according to the frequency and the intensity of the signal.

The strength of the carrier and the degree of phase modulation may be increased by amplifiers and frequency multipliers respectively in other stages of the transmitter before it is radiated from the antenna. A limiting stage D may succeed the phase modulator stage C to eliminate any amplitude modulation that may be present. The purpose of resistors 4 and 5 in the phase the branch circuit and means for varying said variable impedance in accordance with said modulating potentials.

11. In a phase modulation system, a source of oscillating potentials of carrier wave frequency, a source of modulating potentials, an electron discharge device having an anode, a cathode and a control electrode, a parallel tuned circuit of high impedance to said oscillating potentials, a connection between one end of said parallel tuned circuit and the cathode of said tube, a connection between the other end of said parallel tuned circuit and the control electrode oi said tube, a utilization circuit connected to the anode and cathode of said tube, a branch circuit, an impedance variable in accordance with said modulating potentials in each branch of said branch circuit, phase shifting reactances of different character coupling said source of oscillating potentials to each branch of said branch circuit, a connection between a point intermediate the ends of said parallel tuned circuit and a point on said branch circuit, said point of connection on said parallel tuned circuit and connections between the parallel tuned circuit and tube electrodes comprising means for matching the impedance between the tube electrodes to the impedance of the branch circuit and means for varying said variable impedance in accordance With said modulating potentials.

GEORGE L. USSELMAN. 

